As an important organic acid, lactic acid is widely applied in food, chemistry, and pharmaceutical industries. The most important and extensive application of lactic acid is as a monomer for the synthesis of poly(lactic acid), which has good biocompatibility and biodegradability, and is considered as the most promising renewable material of the 21th century. Lactic acid is a chiral molecule, and its optical activity is bringing by a chiral center in the molecule. According to the optical rotation, lactic acid products can be separated into three classes, L-lactic acid, D-lactic acid, and racemic lactic acid. Owing to an extensive application of poly(L-lactic acid), the production of optically pure L-lactic acid had gained general attention from the beginning.
There are three major methods to produce lactic acid: chemical synthesis, enzymatic method, and biotechnological production. Compared to chemical and enzymatic methods, microbial fermentation has significant advantages, i.e., lactic acid can be produced from decomposition products such as glucose from starch and cellulose, which are renewable resources. In addition, an optically pure product of L-lactic acid or D-lactic acid or their mixtures in different proportions can be obtained through different fermentation methods with various strains. Because of the low cost and high safety, microbial fermentation is the major method for lactic acid production in large scale. According to the Chinese patent 200480036931.1, Lactobacillus, Streptococcus, Enterococcus, Rhizopus, etc., have the ability to produce lactic acid. Thermophilic B. coagulans is a new kind of strain that can be used for L-lactic acid production. In recent years, studies on the production of lactic acid by Bacillus have increased gradually due to the advantages of Bacillus strains, e.g., low nutrition requirement and high fermentation temperature that allows open fermentation associated with sharply lowered bacterial pollution during production courses and enhances the optical purity of L-lactic acid. Lactic acid produced with Bacillus strains were reported in the Japanese patents JP5840093, JP606200, and JP327291, the U.S. Pat. No. 5,079,164, and the Chinese patent 200810098908.5.
According to a search through previous literature, L-lactic acid production using B. coagulans from glucose were reported in the Chinese patents 200710176060.9, 200910028930.7, and 02806664.2. However, of all those B. coagulans strains, the ability of producing high amounts of lactic acid from pentoses such as xylose has not been shown. Especially in the patent “A method for L-lactic acid production and its exclusive strain of B. coagulans,” the B. coagulans strain that has been used cannot utilize xylose according to the description.
A further search revealed that there are several B. coagulans strains that can produce L-lactic acid from glucose and xylose (US patent US 2005/0250192 A1). Among those separated strains, Bacillus sp. 36D1 possesses the strongest ability of utilizing hexose and pentose. However, low amounts of L-lactic acid could be produced by Bacillus sp. 36D1. For example, about 25.2 g/L L-lactic acid was produced by Bacillus sp. 36D1 from glucose and about 23.4 g/L L-lactic acid was produced by Bacillus sp. 36D1 from xylose. The highest amount of L-lactic acid was 55.5 g/L produced from reducing sugars (mainly pentose-xylose, hexose-glucose) in sugar cane bagasse with a long fermentation time (over 190 hours).
Currently, the main raw materials for industrial production of lactic acid are glucose, maize, rice, etc., that contain high levels of starch. The main problem using these materials is the high costs. Lactic acid fermentation using organic wastes containing carbohydrates could not only lower the production costs, but also solve the problems of waste resourcization. As a part of the worldwide attempts of protecting the environment and reducing the energy demand, the development of techniques that efficiently recycle organic wastes is underway here and abroad. Organic wastes contain pentoses (xylose, arabinose, etc.) and hexoses (glucose, etc.). However, most of the lactic acid-producing strains cannot use pentoses, which limits the application of organic wastes in lactic acid production. In China, parts of the commercially available xylitol are made from xylose, which is extracted from corncob hydrolyzate. In addition, during that process, many byproducts containing 50%-70% carbohydrates are generated, which will result in high amounts of waste and pollution if not recycled.